1. Field of the Invention
The present invention relates to an interface system for a servo controller, between a servo system for driving a load such as a machine tool with a plurality of servomotors and a numerical control apparatus.
2. Description of the Related Art
In the above-mentioned field, servomotors are driven by command signals from a numerical control apparatus (hereinafter referred to as an "NC apparatus") or the like for accurately controlling a multi-axis machine tool or the like. Such servomotors have conventionally been controlled by an analog servo systems. With the recent advance of computerized control technology, various efforts have been made for efficiently controlling a plurality of servomotors with a single control apparatus.
FIG. 2 of the accompanying drawings is a block diagram of a servomotor control circuit and driver circuit. A control circuit 1 on the side of an NC apparatus is connected to a system bus line of a servo control microprocessor (servo CPU) so that servo system operation can be determined. The control circuit 1 comprises a memory 2 for reading in a position command, an arithmetic unit 3 for processing the position command and a present position signal that is fed back, and a positioning controller 4 for issuing a speed command VCMD based on the result of the arithmetic operation. A speed control circuit 5 on the side of a servomotor includes an arithmetic unit 6 for processing the speed command VCMD and a speed signal which is fed back, a speed controller 7, a current loop processor 8, a current controller 9, and an alarm processing circuit 10. Pulse width modulation (PWM) signals from the current controller 9 are supplied through an insulated amplifier 11 to an inverter 12 composed of a transistor bridge for driving a servomotor 13 with a determined drive current.
Designated at 14 is a detector such as a rotary encoder for detecting the speed and the present position of the servomotor.
The control circuit 1 receives data such as a position command from the servo CPU connected through a common RAM to a main CPU in the form of a microprocessor which effects sequential processing, and is also supplied with the speed and present position data fed back from the servomotor, through an interface circuit coupled to a data bus of the servo CPU.
The speed control circuit 5, which drives the inverter 12 connected to the servomotor 13, effects prescribed feedback control for a current and a speed on the servomotor 13. There are as many servomotors 13 as the number of the axes of the machine tool to be controlled. Where DC servomotors are employed, four transistors of the driver circuit shown in FIG. 3 are driven by the PWM signals A through D from the speed control circuit 5.
With the aforesaid conventional servo controller interface system, there are required as many speed control circuits 5 as the number of the axes of the machine tool which are to be servo-controlled. For highly accurate servo control, data should be transferred at a high speed between the NC apparatus and the speed control circuit 5 for the servo motor. Therefore, the interface circuit is complex out of necessity, and the maintenance of the hardware in general, and particularly that connected to the servomotor is difficult.